Internal combustion engines may be configured to combust hydrocarbon fuel to produce power. However, combustion of hydrocarbon fuel produces undesirable pollutants, such as nitrogen oxide and nitrogen dioxide, collectively referred to as NOx. Various environmental regulations have resulted in a need to significantly reduce the levels of pollutants without restricting the performance of an engine. One method of reducing the NOx emission is using lean burn technology. Generally, with a leaner mixture of air and fuel (i.e., a mixture having a relatively low amount of fuel compared to the amount of air), an engine may produce less NOx emission. Unfortunately, some fuels, such as natural gas, may experience unstable combustion at lean air-to-fuel ratios, due to the increased coefficient of variation (COV) of the indicated mean effective pressure (IMEP) and the increased risk of engine misfire. For example, excessive lean mixture in the vicinity of a spark plug at the time of spark discharge increases the risk of misfire, resulting in increased exhaust emissions and decreased thermal efficiency. Therefore, such combustion instability may prevent the lean burn technology from being used to reduce the NOx emissions to a satisfaction level.
To improve the combustion stability under lean burn conditions, the hydrocarbon fuel may be enriched with hydrogen (H2). For example, a reformer may be used to reform at least part of the fuel into a synthesis gas (syngas), mainly including H2 and carbon monoxide (CO). The syngas is then mixed with the air and unreformed fuel to form a H2-rich mixture, which has improved ignition properties and therefore may help reduce the NOx emissions. The amount of syngas required to meet low NOx emissions may depend on the fuel quality and engine operating conditions. Accordingly, accurate sensing and control of the composition of the fuel and of operating characteristics of the engine enables improved engine performance and reduced production of pollutants.
Traditionally, the reformer and the engine are operated as independent entities. A controller of the engine determines the required amount of syngas as a function of the NOx emission target of the engine, and sends signals to the reformer requesting the determined amount of syngas. The reformer then independently tries to generate the requested amount of syngas. However, such a rigid control scheme does not work well under changing fuel qualities and engine operating conditions. For example, as the fuel quality changes, the performance and the NOx emissions of the engine may deteriorate even if the reformer is generating the requested amount of syngas.
One attempt to address the above-described problems is disclosed in Patent Application Publication No. 2011/0296844 (the '844 publication) by Widener et al., published on Dec. 8, 2011. In particular, the '844 publication discloses a gas turbine engine system utilizing a non-catalytic fuel reformer to partially oxidize a portion of the fuel stream feeding the gas turbine. The reformer improves combustion performance, such as flame stability and emissions, by doping the fuel with small amounts of hydrogen and/or carbon monoxide. The reformer is in operative communication with an engine control system. The engine control system employs various sensors to monitor the reactivity of the fuel and conditions within the gas turbine system, such as ambient temperature, humidity, exhaust backpressure, exhaust emissions, etc. The engine control system is configured to regulate fuel flow to the reformer and control the percentage of fuel reformed based on the monitored parameters. When certain parameters reach a predetermined target, the engine control system may alter the portion of the fuel being reformed or even momentarily cease reforming altogether.
Although the system described in the '844 publication may be useful in providing the amount of syngas required to meet a NOx emission target, it may be less than optimal. For example, the system does not enable a close correlation between the control of the reformer and the NOx emissions of the gas turbine. Therefore, the system may not readily meet the desired NOx emission level under changing fuel qualities and gas turbine conditions. Moreover, the system relies on a determination of both the fuel qualities and the gas turbine conditions. This not only adds to the complexity of the control scheme, but also results in relatively slow system response times dependent upon measurement of different characteristics of the fuel and the gas turbine. The speed of the system may preclude its use in highly-transient applications (for example, in combustion engine applications). In addition, the gas turbine is fundamentally different from a reciprocating internal combustion engine. For example, the combustion dynamics in a gas turbine are mostly constant, whereas the in-cylinder pressure in a reciprocating engine varies through different strokes of the engine, which make it more difficult to coordinate the operations of the reformer and the reciprocating engine.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems existing in the art.